Automatically controlled cleaning arrangement in particular for a vehicle windshield

ABSTRACT

An automatically controlled cleaning device (14) for removing visibility-impairing foreign matter, e.g. rain, snow or mud, from the surface of a window such as a motor vehicle windscreen (8). The device uses a sensor such as an ultrasonic transducer (12a, 12b) in which the frequency (f) and the spacing (d) between the transmitter (12a) and the receiver (12b) enable the presence of foreign matter to be detected simply by measuring the phase difference between the transmitted signal and the received signal. This device may be used in automatic wiper systems or the like.

FIELD OF THE INVENTION

The present invention concerns generally an automatically controlledcleaning arrangement intended for the elimination of foreign matter suchas rain, snow or mud which hinders visibility from a glass surface suchas a vehicle windshield and, more specifically, it concerns anarrangement of this type employing elastic waves in order to detect thepresence of such foreign matter.

BACKGROUND OF THE INVENTION

Different methods for detecting the presence of foreign matter on aglass surface are known. In most cases, advantage is taken of the factthat the transmission of a wave (optical, elastic or other wave) in thematerial of the glass can be found to be noticeably affected by amodification of the medium in which the glass is located. Thus, in thecase of a windshield for an automotive vehicle, the transmission of aluminous wave by multiple reflection in the thickness of the glass maybe more or less attenuated as a function of the surface state of one ofthe boundary surfaces of the windshield.

French patent 2.648.096 proposes an automatic cleaning system usingoptical detection means applying this principle of attenuation.

This latter principle still remains valid in the case of an elastic waveand ultrasonic detection systems have often been proposed.

German patent DE 40 33 975, for example, detects the variations inamplitude of an ultrasonic signal received at a detector arranged at acertain distance from the transmitter.

An analogous principle of detection by amplitude variation is employedin the French patent 91 05784 in which the functions of transmission andreception are effected by one and the same transducer.

In Japanese patent application JP 59-192 651, it is the existence ofsupplemental pulses in the ultrasonic signal received by the detectorwhich enables deduction of the presence of foreign bodies. Suchadditional pulses arise from the fact that the presence of foreignbodies generates supplemental wave transmission paths.

The known arrangements cited hereinabove employing an ultrasonicdetection system nevertheless exhibit the drawback of operating at arelatively high frequency, typically on the order of 1 to 100 MHz; andwhether such frequency be emitted as a continuous wave or in the form ofpulses, the processing circuits must also be able to work at highfrequency. This condition forbids envisaging for such applications theuse of inexpensive circuits such as those formed with the help of slowertechnologies, for example MOS or CMOS technologies. Now it is readilyconceived that a high cost of obtaining such circuits constitutes abrake on the generalization of use of such automatically controlledcleaning arrangements which represent however, for the user, anundeniable advantage as much in view of comfort as that of security.

Thus a purpose of the invention is to obtain an automatically controlledcleaning arrangement intended for the elimination of foreign matter fromthe surface of a glass using an ultrasonic detector and not showing thedrawbacks mentioned hereinabove.

Another purpose of the invention is to obtain a cleaning arrangement thedetector of which functions at a relatively low frequency.

Still another purpose of the invention is to obtain a cleaningarrangement the cost of which is lower than that of systems of the priorart.

SUMMARY OF THE INVENTION

Such purposes can be attained thanks to the fact that it has beenobserved that, for a given frequency and distance between thetransmitter and the receiver, the phase difference between the signalemitted by the transmitter and the signal received by the receiver canvary, in particular, as a function of the presence of foreign matter onthe surface of the glass. This phenomenon is observed for an emittedsignal frequency on the order of 100 kiloHertz.

The characteristics of the invention are defined in the attached claims.

Other purposes, characteristics and advantages of the present inventionwill appear more clearly upon reading the following description of anembodiment thereof, said description being intended to be purelyillustrative and in relation with the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic of the principle of the arrangement of theinvention;

FIG. 2 is a partial cross-section showing the assembly of thepiezoelectric transducers on the windshield;

FIG. 3 shows a piezoelectric pellet provided with its electrodes, and

FIG. 4 shows a schematic block of the control circuit for the windshieldwiper motor.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The description of the present invention which follows will beestablished within the framework of an application for the eliminationof moist foreign matter such as mud, rain, snow, etc. from thewindshield of an automotive vehicle.

At the same time, it is self-evident that the invention is in no mannerlimited to this application and that it may be advantageously employedwithin the framework of any other cleaning application for a thin sheetformed of a material capable of transmitting an ultrasonic signal.

Referring to FIG. 1, there is seen a standard arrangement for operatinga windshield wiper designated by the general reference 1. Arrangement 1comprises windshield wipers 2 mechanically coupled to a motor 4 suchthat when they are activated, they sweep a zone 6 in the form of acircular sector of a single layer windshield 8. Activating means 10connected to motor 4 enable the placing in operation and/or stopping ofsuch motor.

The automatic operation of windshield wipers according to the inventionis brought about with the help of electrosonic detection means 12a and12b placed against the windshield facing zone 6 and from a controlcircuit 14, the function and structure of which are described inrelation with FIGS. 2 to 4.

Referring now to FIGS. 2 and 3, there will be seen respectively thepositioning of transducers 12a and 12b on the windshield 8 as well asthe form of the electrodes of transducers 12a, 12b.

The transducers, whether they be used in transmission or reception, areidentical. Transducers 12a, 12b comprise a piezoelectric element 16,electrodes 18, 20 as well as damping means 22. They are secured onto theinterior face of windshield 8 by means of a thin film of glue 24represented by a stroke on FIG. 2. The two transducers 12a, 12b areseparated by a distance d the determination of the value of which willbe explained subsequently.

Transducers 12a, 12b may be made in the form of pellets of apiezoelectric material such as PZT, lithium niobate, lead titanate, etc.Each transducer 12a, 12b comprises a first electrode 18 (for instancenegative) which extends on the one hand over its entire face which is incontact with the windshield 8 and, on the other hand, on a portion ofthe opposite face (FIG. 3). These transducers 12a, 12b each comprise asecond electrode 20 (positive) insulated from the first electrode 18 andwhich covers over the portion of said opposite face not covered over bythe first electrode 18.

The electrodes are formed by known means, such as the deposition andchemical engraving of a metal or of a metallic alloy (nickel, silver,etc.). The damping means 22 are assembled in an appropriate manner onthe piezoelectric element 16. The role of such damping means 22 is, onthe one hand, to confine the mechanical and vibration energy in thedesired direction (that is to say, towards the windshield) and, on theother hand, to attenuate the reflected interference vibrations of thesignal transmitted and/or received by the transducers.

Information on the constitution and assembly of such damping means 22may be found respectively in the article of Y. Bar-Cohen et al.appearing in the review J. Acoustic Am. Vol. 75, Nr. 5, May 1984 onpages 1629 et following and in the French patent application 91 05784.

FIG. 4 shows the block schematic of the control circuit 14 of thecleaning arrangement of the invention. Circuit 14 comprises a generator26 for a periodic signal having the desired frequency (see after).Generator 26 is coupled, on the one hand, to the transmitting transducer12a and, on the other hand, to the input 28 of a phase detection circuit30. An amplifier 32 which receives as an input the signal provided bytransducer 12b provides an amplified signal at a second input 34 of thephase detection circuit 30. A comparator circuit 36 receives on a firstinput 38 the output signal of said phase detection circuit 30. Itssecond input 40 is coupled to means 42 for setting a threshold value.The output of the comparator circuit controls the activating means 10 ofthe windshield wiper motor 4.

The principle of operation of the arrangement is as follows. Thus, ashas been said hereinabove, there exists for a given windshield acorrelation between the frequency (f) of the signal and the distance (d)between the transmitter and the receiver for which the phase differencebetween the emitted signal and the received signal varies considerablyas a function of the surface state of the windshield. The frequency ison the order of 100 to 150 kiloHerz and the distance d on the order of10 to 20 millimeters. It will be noted that the dimensions have beenhighly exaggerated on the drawing for reasons of clarity. The exactvalues of the frequency and of the distance depend on the constitution(material, mono- or multilayer, etc.) and on the thickness of thewindshield and also on those of the transducers. The determination ofthe couple (f)/(d) may be obtained in an empirical manner. For that itis necessary to have available a generator for variable frequenciescovering the range from 100 to 150 kiloHertz and adjustement means forthe distance d. Such phase difference is that which exists between theemitted wave and the resulting wave detected by the receiver.

The waves which are propagated between the transmitter and the receiverare subject to multiple reflections and trace different trajectories,certain of which have been shown by S on FIG. 2. Some of these waves areinfluenced by the presence of water, for example by a modification ofthe amplitude thereof. Certain other waves are not influenced, as forexample the direct wave or again the wave which is propagated in theinterior sheet of a layered windshield.

The received signal is determined by the vector sum of the differentacoustic waves which are propagated between the transmitter and thereceiver.

The adjustment consists in selecting a pair frequency--distance which inthe vector sum renders preponderant the participation of waves sensitiveto the presence of water on the outer surface of the windshield.

If reference is made to the schematic drawing of FIG. 4, generator 26emits a continuous electrical signal at the desired frequency which isapplied to the electrodes of transducer 12a. This latter generates anelastic vibration of the same frequency which is transmitted to thewindshield on which it is secured. Transducer 12b transforms thereceived vibration into an electrical signal which, followingamplification, is applied to the phase detector 30. Such phase detectorfurnishes at its output a signal representing the phase differenceexisting between the signal emitted by generator 26 and the signalissuing from amplifier 32. If the value of the phase difference exceedsa threshold value determined by means 42 (the value of the threshold maybe determined in an empirical manner according to the determination ofthe frequency and distance d), the output of comparator 36 changes itslogic state and operates the activator 10 of the windshield wiper motor.It is evident that in place of an order "all or nothing" for thewindshield wiper motor, one could have the speed of such motor depend onthe signal representing the phase difference which is of an analognature. Thus, the speed of the motor would be directly related with thequantity of impurities (rain, snow, mud, etc.) on the surface of thewindshield.

Although the present invention has been described within the frameworkof a specific embodiment, it is however clear that it is not limited tosuch example and that it is capable of modifications or variants withoutdeparting from its domain.

We claim:
 1. Automatically controlled cleaning arrangement intended forthe elimination of moist foreign bodies from a glass surface andcomprising cleaning means for said surface, activating means for saidcleaning means, means for detecting the presence of foreign bodies withthe help of an ultrasonic transmitter and receiver arranged on the glassin order to furnish a detection signal representing such presence andcontrol means for operating said activating means in response to thedetection signal, characterized in that the frequency (f) of the signalemitted by the transmitter and the distance (d) existing between thetransmitter and the receiver are selected in a manner such that thephase difference between said emitted signal and the signal received bythe receiver is a maximum in the presence of foreign bodies and in thatsaid control means includes a circuit for measuring said phasedifference.
 2. Automatically controlled cleaning arrangement accordingto claim 1 characterized in that said signal emitted by the transmitteris a continuous wave signal.
 3. Automatically controlled cleaningarrangement according to claim 1 characterized in that the frequency ofthe emitted signal is comprised between 100 and 150 kiloHertz and inthat the distance between the transmitter and the receiver is comprisedbetween 10 and 12 millimeters.
 4. Automatically controlled cleaningarrangement according to any of claims 1 to 3 characterized in that saidtransmitter and said receiver are piezoelectric transducers. 5.Automatically controlled cleaning arrangement according to claim 4characterized in that said control means comprise:a generator forgenerating electrical signals at the frequency f, the output of which isapplied to said transmitter; an amplifier for amplifying the signalfurnished by said receiver; a circuit for measuring the phase differencebetween the signal furnished by said generator and the signal furnishedby said amplifier; and a comparator circuit for comparing the value ofsaid phase difference with a threshold value and operating saidactivating means whenever said phase difference is greater than saidthreshold value.
 6. Automatically controlled cleaning arrangementaccording to claim 2 characterized in that the frequency of the emittedsignal is comprised between 100 and 150 kiloHertz and in that thedistance between the transmitter and the receiver is comprised between10 and 20 millimeters.
 7. Automatically controlled cleaning arrangementaccording to claim 3 characterized in that said control means comprise:agenerator for generating electrical signals at the frequency f, theoutput of which is applied to said transmitter; an amplifier foramplifying the signal furnished by said receiver; a circuit formeasuring the phase difference between the signal furnished by saidgenerator and the signal furnished by said amplifier; and a comparatorcircuit for comparing the value of said phase difference with athreshold value and operating said activating means whenever said phasedifference is greater than said threshold value.
 8. Automaticallycontrolled cleaning arrangement according to claim 1 characterized inthat said control means comprise:a generator for generating electricalsignals at the frequency f, the output of which is applied to saidtransmitter; an amplifier for amplifying the signal furnished by saidreceiver; a circuit for measuring the phase difference between thesignal furnished by said generator and the signal furnished by saidamplifier; and a comparator circuit for comparing the value of saidphase difference with a threshold value and operating said activatingmeans whenever said phase difference is greater than said thresholdvalue.